1. Field of the Invention
The invention relates generally to the field of instruments used to measure the diameter of a wellbore drilled through earth formations. More specifically, the invention relates to methods for calculating a wellbore diameter using measurements made by a gamma-gamma density logging instrument.
2. Description of Related Art
Wellbores are drilled through earth formations for, among other purposes, extracting oil, gas, and water. Wellbores are typically drilled using a rotary drill bit turned by a drilling rig, hydraulically operated motor (xe2x80x9cmud motorxe2x80x9d) or similar devices known in the art. After a wellbore is drilled through the intended earth formations, a protective pipe or casing is typically cemented into the wellbore to maintain the mechanical integrity of the wellbore and to hydraulically isolate the penetrated earth formations from each other. When cementing the casing in place, it is useful to have knowledge of the shape of the wellbore, particularly its diameter along its length, so that the volume of cement needed to fill an annular space between the wellbore wall and the casing can be accurately determined. Various types of caliper devices are known in the art for determining the diameter of the wellbore, such as contact arm devices, and acoustic calipers. A typical contact arm device which can measure the diameter of the wellbore along its length is described in U.S. Pat. No. 3,321,625 issued to Wahl.
More recently it has become common to measure petrophysical properties of the earth formations penetrated by wellbores, called xe2x80x9cloggingxe2x80x9d the wellbore, while the drilling of the wellbore is in progress. See, for example, U.S. Pat. No. 5,513,528 issued to Holenka et al. which describes a method and apparatus for making petrophysical measurements during the drilling process. Such xe2x80x9clogging while drillingxe2x80x9d (LWD) instruments and methods include those which can make measurements corresponding to the bulk density of the earth formations penetrated by the wellbore. One such instrument is described, for example, in U.S. Pat. No. 5,473,158 issued to Holenka et al. One practical limitation of LWD instruments and methods is that using contact arm-type caliper devices to measure wellbore diameter, such as the one disclosed in the Wahl ""625 patent, is extremely difficult and expensive. Consequently, the typical caliper device known in the art is generally an acoustic travel time measurement device, such as disclosed in the Holenka et al ""528 patent referred to earlier. One practical limitation to acoustic travel time measurement devices used in wellbore diameter measurement is that it is necessary to know the acoustic velocity of a liquid (xe2x80x9cdrilling mudxe2x80x9d) filling the wellbore in order to be able to convert acoustic travel time into accurate determinations of distance of the acoustic sensor to the wellbore wall. As is known in the art, a number of factors affect the acoustic velocity of drilling mud, including ambient temperature and pressure. These factors may not be constant along the length of the wellbore. In addition, the acoustic sensor adds cost and complexity to the LWD instrument assembly.
It is desirable to have a method and apparatus for measuring wellbore diameter which does not rely on acoustic travel time through wellbore fluid.
One aspect of the invention is a method for determining the standoff of a well logging instrument from the wall of a wellbore. The method according to this aspect of the invention includes measuring a compensated gamma-gamma formation density at a rotary orientation proximate a bottom of the wellbore, and measuring an apparent density at one other rotary orientation. The apparent density is related to the compensated density, a density of a fluid disposed between the logging instrument and a wall of the wellbore, and the standoff.
Another aspect of the invention is a method for determining a shape of a wellbore. A method according to this aspect of the invention includes measuring a compensated gamma-gamma formation density at a rotary orientation proximate a bottom of the wellbore. An apparent density is then measured at one other rotary orientation about the wellbore. The apparent density is related to the compensated density, a density of a fluid disposed between the logging instrument and a wall of the wellbore, and the standoff. The standoff is then combined with a diameter of a well logging instrument used to make the gamma-gamma density measurements to determine the shape of the wellbore.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.